In a large pot of lightly salted boiling water, cook the fettuccine according to package directions.
Drain and transfer to a serving bowl.
Pour lemon juice and olive oil over pasta, toss lightly, and set aside.
Meanwhile, in a large skillet over medium-high heat, heat the diced tomatoes to near boiling and add fish fillets.
Cook about 5 to 7 minutes, until fish is cooked through and flakes easily with a fork. Break apart the fish fillets into small chunks.
Add the tomato/fish mixture to the pasta, toss again, and serve.